Siccar Point Research Articles

Ice? Salt? Pressure? Sediment deformation structures as evidence of late-stage shallow groundwater in Gale crater, Mars

Persistence of near-surface water during the late evolution of Gale crater, Mars, would have been fundamental for maintaining a habitable environment. Sedimentation in aqueous conditions is evident during the early stages of crater infilling, where accumulation of lower Mount Sharp group strata is characterized by fluviolacustrine sedimentary rocks. The basal unit of the Siccar Point group—the Stimson formation—which unconformably overlies the Mount Sharp group and represents conditions postdating the exhumation of Aeolis Mons, is characterized by accumulation of aeolian strata under arid conditions. Water was largely absent near the surface during its deposition. At the Feòrachas outcrop, discovery of soft sediment deformation structures in aeolian Stimson strata challenges the notion that Gale crater was devoid of water during its later depositional phase. We identified deformed wind-rippled and vertically laminated sandstones, hosted within erosion-resistant ridges forming boxwork patterns. Broadly, these structures are diagnostic of water (as liquid or as ice) in the shallow subsurface. Comparison with Earth analogues suggests formation by subsurface fluid escape, freeze-thaw processes, or evaporite deformation. Regardless of the mechanism, these structures signify the presence of water at or near the surface much later than previously documented and may extend the habitability window in Gale crater.

The Curiosity Rover's Exploration of Glen Torridon, Gale Crater, Mars: An Overview of the Campaign and Scientific Results.

The Mars Science Laboratory rover, Curiosity, explored the clay mineral-bearing Glen Torridon region for 1 Martian year between January 2019 and January 2021, including a short campaign onto the Greenheugh pediment. The Glen Torridon campaign sought to characterize the geology of the area, seek evidence of habitable environments, and document the onset of a potentially global climatic transition during the Hesperian era. Curiosity roved 5km in total throughout Glen Torridon, from the Vera Rubin ridge to the northern margin of the Greenheugh pediment. Curiosity acquired samples from 11 drill holes during this campaign and conducted the first Martian thermochemolytic-based organics detection experiment with the Sample Analysis at Mars instrument suite. The lowest elevations within Glen Torridon represent a continuation of lacustrine Murray formation deposits, but overlying widespread cross bedded sandstones indicate an interval of more energetic fluvial environments and prompted the definition of a new stratigraphic formation in the Mount Sharp group called the Carolyn Shoemaker formation. Glen Torridon hosts abundant phyllosilicates yet remains compositionally and mineralogically comparable to the rest of the Mount Sharp group. Glen Torridon samples have a great diversity and abundance of sulfur-bearing organic molecules, which are consistent with the presence of ancient refractory organic matter. The Glen Torridon region experienced heterogeneous diagenesis, with the most striking alteration occurring just below the Siccar Point unconformity at the Greenheugh pediment. Results from the pediment campaign show that the capping sandstone formed within the Stimson Hesperian aeolian sand sea that experienced seasonal variations in wind direction.

Alteration at the Base of the Siccar Point Unconformity and Further Evidence for an Alkaline Provenance at Gale Crater: Exploration of the Mount Sharp Group, Greenheugh Pediment Cap Rock Contact With APXS

AbstractChemical data acquired by Curiosity's Alpha Particle X‐ray Spectrometer during examination of the contact between the upper Mount Sharp group and overlying Stimson formation sandstones at the Greenheugh pediment reveal compositional similarities to rocks encountered earlier in the mission. Mount Sharp group strata encountered below the Basal Siccar Point group unconformity at the base and top of the section, separated by >300 m in elevation, have distinct and related compositions. This indicates enhanced post‐depositional fluid flow and alteration focused along this contact. Sandstone targets exposed immediately above the unconformity have basaltic compositions consistent with previously encountered eolian Stimson formation sandstones, except at the contact, where they show the addition of S. Resistant sandstone outcrops above the contact have higher K, Mn, and Na and lower Ni concentrations that primarily reflect changes in provenance. They are compositionally related to cap rock float blocks encountered as Curiosity climbed through the Mount Sharp group, and Bradbury group sandstone outcrops. The higher K, pediment sandstones are interpreted to have a similar provenance to some Bradbury group sandstones, further evidence for widespread, alkaline source rock within and/or in the vicinity of Gale crater. The Bradbury and Siccar Point groups may both be younger than the Mount Sharp group. Alternatively, an alkaline source area in and around Gale crater has been eroded by both water and wind at different times (both before and after deposition of the Mount Sharp group), during the evolution of the crater and its infill.

Constraining Alteration Processes Along the Siccar Point Group Unconformity, Gale Crater, Mars: Results From the Sample Analysis at Mars Instrument

AbstractResults from the Sample Analysis at Mars‐evolved gas analyzer on board the Mars Science Laboratory Curiosity rover constrained the alteration history and habitability potential of rocks sampled across the Siccar Point unconformity in Gale crater. The Glasgow member (Gm) mudstone just below the unconformity had evidence of acid sulfate or Si‐poor brine alteration of Fe‐smectite to Fe amorphous phases, leaching loss of Fe‐Mg‐sulfate and exchange of unfractionated sulfur 34S (δ34S = 2‰ ± 7‰) with enriched 34S (20‰ ± 5‰, Vienna Cañon Diablo Troilite). Carbon abundances did not significantly change (322–661 μgC/g) consistent with carbon stabilization by amorphous Al‐ and Fe‐hydroxide phases. The Gm mudstone had no detectable oxychlorine and extremely low nitrate. Nitrate (0.06 wt.% NO3), oxychlorine (0.13 wt.% ClO4), high C (1,472 μg C/g), and low Fe/Mg‐sulfate concentration (0.24 wt.% SO3) depleted in 34S (δ34S = −27‰ ± 7), were detected in the Stimson formation (Sf) eolian sandstone above the unconformity. Redox disequilibrium through the detections of iron sulfide and sulfate supported limited aqueous processes in the Sf sandstone. Si‐poor brines or acidic fluids altered the Gm mudstone just below the unconformity but did not alter underlying Gm mudstones further from the contact. Chemical differences between the Sf and Gm rocks suggested that fluid interaction was minimal between the Sf and Gm rocks. These results suggested that the Gm rocks were altered by subsurface fluids after the Sf placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations.

Statistical Analysis of APXS‐Derived Chemistry of the Clay‐Bearing Glen Torridon Region and Mount Sharp Group, Gale Crater, Mars

AbstractThe Glen Torridon stratigraphic sequence marks the transition from the low energy lacustrine‐dominated Murray formation (Mf) (Jura member: Jm) to the more diverse Carolyn Shoemaker formation (CSf) (Knockfarril Hill member: Knockfarril Hill; Glasgow member: Glasgow). This transition defines a change in depositional setting. Alpha Particle X‐ray Spectrometer (APXS) results and statistical analysis reveal that the bulk primary geochemistry of Mf targets are broadly in family with CSf targets, but with subtle compositional and diagenetic trends with increasing elevation. APXS results reveal significant compositional differences between Jura_GT and the stratigraphically equivalent Jura on Vera Rubin ridge (Jura_VRR). The data define two geochemical facies (high‐K or high‐Mg), with a strong bimodal grain distribution in Jura_GT and Knockfarril Hill. The contact between Knockfarril Hill and Glasgow is marked by abrupt sedimentological changes but a similar composition for both. Away from the contact, the Knockfarril Hill and Glasgow plot discretely, suggesting a zone of common alteration at the transition and/or a gradual transition in provenance with increasing elevation in the Glasgow member. APXS results point to a complex history of diagenesis within Glen Torridon, with increasing diagenesis close to the Basal Siccar Point unconformity on the Greenheugh pediment, and with proximity to the beginning of the clay sulfate transition. Elemental mobility is evident in localized enrichments or depletions in Ca, S, Mn, P, Zn, Ni. The highly altered Hutton interval, in contact with the unconformity on Tower butte, is also identified on Western Butte, indicating that the “interval” was once laterally extensive.

E&P Notes (January 2022)

E&P Notes (January 2022)

Origin and composition of three heterolithic boulder- and cobble-bearing deposits overlying the Murray and Stimson formations, Gale Crater, Mars

Heterolithic, boulder-containing, pebble-strewn surfaces occur along the lower slopes of Aeolis Mons (“Mt. Sharp”) in Gale crater, Mars. They were observed in HiRISE images acquired from orbit prior to the landing of the Curiosity rover. The rover was used to investigate three of these units named Blackfoot, Brandberg, and Bimbe between sols 1099 and 1410. These unconsolidated units overlie the lower Murray formation that forms the base of Mt. Sharp, and consist of pebbles, cobbles and boulders. Blackfoot also overlies portions of the Stimson formation, which consists of eolian sandstone that is understood to significantly postdate the dominantly lacustrine deposition of the Murray formation. Blackfoot is elliptical in shape (62 × 26 m), while Brandberg is nearly circular (50 × 55 m), and Bimbe is irregular in shape, covering about ten times the area of the other two. The largest boulders are 1.5–2.5 m in size and are interpreted to be sandstones. As seen from orbit, some boulders are light-toned and others are dark-toned. Rover-based observations show that both have the same gray appearance from the ground and their apparently different albedos in orbital observations result from relatively flat sky-facing surfaces.Chemical observations show that two clasts of fine sandstone at Bimbe have similar compositions and morphologies to nine ChemCam targets observed early in the mission, near Yellowknife Bay, including the Bathurst Inlet outcrop, and to at least one target (Pyramid Hills, Sol 692) and possibly a cap rock unit just north of Hidden Valley, locations that are several kilometers apart in distance and tens of meters in elevation. These findings may suggest the earlier existence of draping strata, like the Stimson formation, that would have overlain the current surface from Bimbe to Yellowknife Bay. Compositionally these extinct strata could be related to the Siccar Point group to which the Stimson formation belongs.Dark, massive sandstone blocks at Bimbe are chemically distinct from blocks of similar morphology at Bradbury Rise, except for a single float block, Oscar (Sol 516). Conglomerates observed along a low, sinuous ridge at Bimbe consist of matrix and clasts with compositions similar to the Stimson formation, suggesting that stream beds likely existed nearly contemporaneously with the dunes that eventually formed the Stimson formation, or that they had the same source material. In either case, they represent a later pulse of fluvial activity relative to the lakes associated with the Murray formation.These three units may be local remnants of infilled impact craters (especially circular-shaped Brandberg), decayed buttes, patches of unconsolidated fluvial deposits, or residual mass-movement debris. Their incorporation of Stimson and Murray rocks, the lack of lithification, and appearance of being erosional remnants suggest that they record erosion and deposition events that post-date the exposure of the Stimson formation.

Extensive Polygonal Fracture Network in Siccar Point group Strata: Fracture Mechanisms and Implications for Fluid Circulation in Gale Crater, Mars

AbstractRock fractures are indicators of stress release within geologic systems, and fracture morphologies can commonly be used to infer formation conditions. Polygonal fractures are common in isotropic, contractional stress regimes such as in rocks exposed at the surface of a planet undergoing thermal cycling or in sedimentary substrates undergoing repeated wetting and drying. Such polygonal fracture systems, on centimeter to decameter scales, have been widely documented on Mars. Utilizing a combination of orbital‐ and ground‐based images, we report a laterally extensive polygonal fracture network that occurs within siliciclastic rocks of the lowermost Siccar Point group, Gale crater, Mars. The Siccar Point group is exposed over approximately 20 km2 in northwest Gale crater, where it unconformably overlies eroded strata of Mount Sharp (Aeolis Mons) and reflects likely aeolian deposition along the lower flanks of Mount Sharp. Images reveal an extensive network of erosionally resistant polygons, approximately 7.5 m across, that exhibit interior angles (i.e., fracture intersections) with modes at 90° and 120°. Polygon morphology indicates that fractures formed during multiple cycles of expansion and contraction, which is attributed to desiccation and subsequent recharge of near‐surface groundwater. The erosional resistance of preserved fractures is inferred to reflect postfracture diagenetic fluid flow along the sub–Siccar Point group unconformity and cementation. Evidence for multiple fluid events in the relatively young strata of the Siccar Point group requires a protracted history of fluid stability in Gale crater.

Classic Rock Tours 1. Hutton’s Unconformity at Siccar Point, Scotland: A Guide for Visiting the Shrine on the Abyss of Time

The angular unconformity at Siccar Point in Scotland is one of the most famous localities in the history of geology. At this spot, steeply dipping, folded turbiditic sandstone of early Silurian age is clearly overlain by subhorizontal red conglomerate, breccia and sandstone of late Devonian age. Siccar Point was not the first unconformity ever to be described or illustrated, but it is unquestionably one of the most spectacular and informative that geologists are likely to see. In June of 1788, a famous excursion by James Hutton, John Playfair and Sir James Hall first discovered this striking evidence for the cyclic nature of geological processes and the probable antiquity of the Earth. Contrary to myth, it was likely not the inspiration for Hutton’s famous phrase no vestige of a beginning, no prospect of an end, but Playfair’s metaphor of looking so far into the abyss of time is forever associated with this place. Siccar Point influenced many other geologists, including the young Charles Lyell, who would eventually bring the ideas of James Hutton together with those of William Smith, to build the uniformitarian paradigm that founded modern geology. Lyell’s writings would in turn influence the young Charles Darwin in his search for the reality and causes of evolution. Siccar Point is easy to visit from the historic and vibrant city of Edinburgh, and such a pilgrimage is easily combined with other sights of geological or cultural interest. Visiting the shrine involves a short coastal hike in one of the most beautiful parts of Scotland. This article combines practical advice for would-be pilgrims to Siccar Point with some historical context about its pivotal role in the development of geological ideas in the enlightenment of the late 18th and early 19th centuries.RÉSUMÉLa discordance angulaire de Siccar Point en Écosse est l'une des localités les plus célèbres de l'histoire de la géologie. À cet endroit, un grès turbiditique plissé à fort pendage du début du Silurien est recouvert de conglomérats rouges subhorizontaux, de brèches et d’un grès de la fin du Dévonien. Siccar Point n'est pas la première discordance qui ait été décrite ou illustrée, mais c'est sans conteste l'une des plus spectaculaires et révélatrices que les géologues puissent voir. En juin 1788, avec leur célèbre excursion, James Hutton, John Playfair et Sir James Hall ont découvert cette preuve frappante de la nature cyclique des processus géologiques et de l`ancienneté probable de la Terre. Contrairement à ce qu'on croit, ce n'est probablement pas la fameuse phrase de Hutton « aucun vestige d'un début, aucune perspective de fin », mais la métaphore de Playfair « voir si loin dans l'abîme du temps » qui est à jamais associée à ce lieu. Siccar Point a influencé de nombreux autres géologues, y compris le jeune Charles Lyell, qui a fini par réunir les idées de James Hutton et celles de William Smith qui ont défini le paradigme uniformitariste, devenu le fondement de la géologie moderne. Les écrits de Lyell influenceront à leur tour le jeune Charles Darwin dans sa recherche de la réalité et des causes de l'évolution. Il est facile de se rendre à Siccar Point depuis cette ville chargée d'histoire et dynamique qu’est Édimbourg, et un tel pèlerinage se combine facilement avec d'autres sites d'intérêt géologique ou culturel. La visite de ce « sanctuaire » implique une courte randonnée côtière dans l'une des plus belles régions d'Écosse. Le présent article combine des conseils pratiques pour les visiteurs potentiels à Siccar Point et présente un historique de son rôle central dans le développement des idées géologiques à la fin du XVIIIe siècle et au début du XIXe siècle.

Assessment of Aeolis Palus stratigraphic relationships based on bench-forming strata in the Kylie and the Kimberley regions of Gale crater, Mars

The stratigraphic context of rock layers is a critical piece of information needed for accurate reconstruction of their geologic history. Although sedimentary rocks are widespread in Gale crater, efforts to deduce stratigraphic relationships of rocks were challenging early in the Mars Science Laboratory mission because vertical bedrock exposures were relatively rare along the first ∼3 km the rover traversed across Aeolis Palus. Potential insights into the three-dimensional configuration of rock layers were made once the rover passed Dingo Gap, especially in the informally-named Kylie and Kimberley regions. Here, the terrain exhibits low relief ( < 10 m) cliffs, some of which are continuous over lengths > 75 m. Curiosity Mastcam and Navcam images show that the cliffs are capped by resistant, bench-forming rock layers corresponding to two facies: a poorly sorted, weakly stratified pebble conglomerate, and a massive, dark-toned, vuggy sandstone. In places, the inclination of the topographic surface (northward ∼2° to 3°) is similar to the apparent dip of the underlying strata, suggesting the presence of dip slopes in an area inferred to be generally flat-lying, conformable rock units. Further, we assessed potential strata correlations via plane-fitting exercises and a regional comparison to other capping strata. We speculate that bench-forming strata in the study region could be part of a widespread package of draping strata (the Siccar Point group) that post-dates deposition and exhumation of the lower strata of Mount Sharp.

Hutton’s Great Unconformity at Siccar Point, Scotland: Where deep time was revealed and uniformitarianism conceived

The classic angular unconformity at Siccar Point became a landmark location in the history of geology after a boat trip to the site by James Hutton and his colleagues Professor John Playfair and Sir James Hall in 1788. Hutton successfully used the unconformity to support his view that the Earth’s landforms and geological record resulted from uniform natural processes such as sedimentation, uplift, erosion, and renewed sedimentation through deep geologic time. At Siccar Point, gently dipping Devonian fluviatile Old Red Sandstones unconformably onlap vertical Silurian deep-water greywackes (schistus) to produce the most classic of angular unconformities that remains a must-see outcrop for geologists to this day. The relevance of this outcrop for the petroleum geologist is that it provides the opportunity to view an end-member stratal relationship in the continuum of unconformable surfaces (i.e., angular unconformity, disconformity, paraconformity, correlative conformity). The locality also aids the visualization of geometries relevant to unconformity (subcrop) traps and to onlap and pinchout (supracrop)–type traps. Although most Scottish geoscientists will have seen this outcrop in their student days, as a teaching aid to illustrate basic principles, a trip later in life can remind us of the importance of the site in a historical geology context. The location continues to inspire and instills a sense of duty in us, as scholars of the Earth’s formation, to communicate and share the principles of deep time with those who have not had the privilege of an education that included the fundamentals of geology.

The stratigraphy and evolution of lower Mount Sharp from spectral, morphological, and thermophysical orbital data sets.

We have developed a refined geologic map and stratigraphy for lower Mount Sharp using coordinated analyses of new spectral, thermophysical, and morphologic orbital data products. The Mount Sharp group consists of seven relatively planar units delineated by differences in texture, mineralogy, and thermophysical properties. These units are (1–3) three spatially adjacent units in the Murray formation which contain a variety of secondary phases and are distinguishable by thermal inertia and albedo differences, (4) a phyllosilicate‐bearing unit, (5) a hematite‐capped ridge unit, (6) a unit associated with material having a strongly sloped spectral signature at visible near‐infrared wavelengths, and (7) a layered sulfate unit. The Siccar Point group consists of the Stimson formation and two additional units that unconformably overlie the Mount Sharp group. All Siccar Point group units are distinguished by higher thermal inertia values and record a period of substantial deposition and exhumation that followed the deposition and exhumation of the Mount Sharp group. Several spatially extensive silica deposits associated with veins and fractures show that late‐stage silica enrichment within lower Mount Sharp was pervasive. At least two laterally extensive hematitic deposits are present at different stratigraphic intervals, and both are geometrically conformable with lower Mount Sharp strata. The occurrence of hematite at multiple stratigraphic horizons suggests redox interfaces were widespread in space and/or in time, and future measurements by the Mars Science Laboratory Curiosity rover will provide further insights into the depositional settings of these and other mineral phases.

Hutton's unconformity at Siccar Point, SE Scotland, UK

Hutton's unconformity at Siccar Point, SE Scotland, UK

MATHEZ, E. A. &amp; WEBSTER, J. D. 2004. The Earth Machine. The Science of a Dynamic Planet. xvi+335 pp. New York: Columbia University Press. Price US $39.95, £27.00 (hard covers). ISBN 0 231 12578 X

MATHEZ, E. A. &amp; WEBSTER, J. D. 2004. The Earth Machine. The Science of a Dynamic Planet. xvi+335 pp. New York: Columbia University Press. Price US $39.95, £27.00 (hard covers). ISBN 0 231 12578 X - Volume 142 Issue 6

Mind over magma: the story of igneous petrology

The book follows the historical course of the development of geological science, from the time of ancient civilisations in the Mediterranean region, up to the present day. However, the beginning of geology as a modern science is generally considered to date from the observation of the Devonian unconformity at Siccar Point near Edinburgh by Hutton around 1788. This followed observations he made at Glentilt of granite cutting the schistositv planes of the schists，

Siccar Point and Teaching the History of Geology

The concept of the rock cycle and the “indefinite” length of geologic time were outlined first by James Hutton before he had ever seen an unconformity. Hutton drew general support of his theory from existing observations of unconformities from accounts in French, which he re-interpreted in light of his own ideas. Hutton then found his own field examples to test his ideas before coming upon Siccar Point. In summing up his evidence, which included wide-ranging observations on the geology of SE Scotland, Hutton himself did not cite Siccar Point. Inaccuracies concerning the role of Siccar Point in the development of James Hutton's ideas remain as entrenched as ever in many physical geology textbooks. A more balanced appraisal of all contributions to Hutton's work is warranted, particularly the contributions of French writers. Finally, an accurate presentation of Hutton's method can contribute to students' understanding of the nature of historical theories and in meeting the challenges of creationists.

Uncomformity at Siccar Point, Southeastern Scotland

Uncomformity at Siccar Point, Southeastern Scotland

New Perspectives on the Old Red Sandstone

The Old Red Sandstone is amongst the most distinctive and well‐known stratigraphic units in the British Isles. It is mainly of Devonian age; in fact, its lower boundary was used to define the base of the Devonian until relatively recently and it was called “Old” back in the nineteenth century to distinguish it from a superficially similar succession of Triassic age named the New Red Sandstone. The Old Red Sandstone has long been known to be a non‐marine syntectonic to post‐tectonic deposit associated with the Caledonian Orogeny One of the most famous outcrops of the red sandstone is at Siccar Point in northeast England at one of several outcrops named “Hutton's unconformity” where it lies, with marked angularity on Silurian lithic sandstones and shales. It was at these outcrops, toward the end of the eigthteenth century that James Hutton first came to understand the meaning of angular unconformities as structures representing vast amounts of missing time during which major upheavals of the Earth's crust occurred.

The Incised Valley and Valley Fill at Siccar Point: ABSTRACT

The Incised Valley and Valley Fill at Siccar Point: ABSTRACT

Unconformity—An Historical Study

It may be assumed that all present-day geologists realise the importance of ‘unconformity’ in its structural and genetic aspects. Although angular unconformity was sketched by Nicolaus Steno in 1669, by John Strachey in 1719 and by Jean Etienne Guettard in 1766, its true significance was not realised until the end of the eighteenth century. The discovery of the real -import of unconformity in geology was made by James Hutton, who ‘needed’ it to substantiate his idea of the cycle of changes in the Earth, the ‘geostrophic cycle’, consisting of a series of processes—denudation, transportation, deposition, consolidation, folding, uplift and so on. Hutton postulated unconformity before observing it, as it was only in 1787 that he discovered a splendid example of angular unconformity on the Isle of Arran, later followed by similar discoveries at Jedburgh and Siccar Point. But Hutton never used the term ‘unconformity’, using instead long descriptive phrases in which the expression ‘conjunction of vertical and horizontal strata’ was widely used. Strange to relate it was the antagonist of Hutton, the celebrated Wernerian, Robert Jameson, who applied the term ‘unconformity’ to the feature discovered by Hutton. This he did in 1805 in his book on the geology of Dumfriesshire, in which he translated the German term proposed by Werner ‘abweichende Lagerung’ (‘deviating bedding’) by an old English ecclesiastical term—‘unconformity’, which originally had the same meaning as ‘nonconformity’. Launched in such a way, the term ‘unconformity’ took a long time to penetrate into the geological vocabulary and it was only after the heroic effort of Charles Lyell and Henry De la Beche in the second quarter of the nineteenth century that the term was universally accepted and understood. Until the beginning of the twentieth century, however, the term ‘unconformity’ meant only ‘angular unconformity’. In 1905 Grabau proposed the term ‘disconformity’, for what was later called ‘parallel unconformity’. In 1909 Blackwelder suggested that ‘the contact between rocks of wholly unlike origin’, such as granite and clay, should also be classed as an unconformity. For this type I would like to suggest the name ‘heterolithic unconformity’. Finally in 1910 Willis proposed that a gap in deposition of sediments should also be considered to represent an unconformity of a type which may be best named ‘non-depositional unconformity’. Thus these four types of unconformity both in their structural aspect and their geochronological aspects are now firmly embedded in the science of geology, as representing a process which played such an important part in the shaping of the visible part of the Earth.